1. Field of the Invention
This invention relates to a method for polymerizing alpha-olefins. More particularly, the invention relates to an improved method of polymerizing alpha-olefins to produce, preferably, linear low density polyethylene and polypropylene, wherein the loss of monomers experienced in prior art processes is greatly reduced.
2. Description of the Prior Art
Polymers and copolymers of C.sub.2 -C.sub.10 olefins, particularly copolymers of ethylene and higher alpha-olefins, have in recent years been produced in gas phase, fluid bed reactors. Karol et al, U.S. Pat. No. 4,302,566, describe a gas phase, fluid bed reactor for producing linear low density polyethylene polymers. Graff, U.S. Pat. No. 4,173,547, Stevens et al, U.S. Pat. No. 3,787,384, Strobel et al, U.S. Pat. No. 4,148,754 and Ziegler, deceased, et al, U.S. Pat. No. 4,063,009, describe various polymerization processes which produce polyethylene other than linear low density polyethylene.
Nowlin et al, U.S. Pat. No. 4,481,301, the entire contents of which are incorporated herein by reference, teach the preparation of a highly active alpha-olefin polymerization catalyst comprising contacting a support material, e.g., silica, containing reactive OH groups with a stoichiometric excess of an organomagnesium composition, and subsequently reacting the product with a tetravalent titanium compound.
Bobst et al, U.S. Pat. No. 4,372,758, disclose a process for removing unpolymerized monomers from olefin polymers, comprising introducing an inert gas into a purge vessel countercurrently to the flow of the product also introduced into the purge vessel. The unreacted hydrocarbon monomers are stripped from the product in the purge vessel, a vent gas containing the monomers is recovered from the purge vessel, a portion of the vent gas is burned in a flare, and the remainder thereof is recycled to the purge vessel as a conveying stream for the polymer or as a purge stream.
As is known to those skilled in the art, heretofore-known gas phase, fluid bed reactor processes for polymerizing alpha-olefins, and especially processes producing linear low density polyethylene polymers, were relatively inefficient because a substantial portion of monomers was not converted to the products and represented lost reactants. There were two principal mechanisms responsible for the loss of alpha-olefins in such a process: (A) reactor gas vented with the polymer in the product discharge system, and (B) unreacted alpha-olefin reactant gas dissolved in the product and carried with it from the reactor downstream into the system. Reactor gases lost by both of these mechanisms were usually removed from the system through a purge bin flare vent. Unreacted monomers usually comprised about 2-8% of the total monomers fed to the reactor.
In previously used gas phase, fluid bed reactor processes, the monomer or monomers were conducted to a fluid bed reactor, normally operating at the pressure of about 300 psig. Simultaneously, but independently of the monomers feed, an olefin polymerization catalyst was also conducted into the fluid bed reactor. The partial pressure of the ethylene monomer in such a gas phase fluid bed reactor was required to be at least 100 psi. At such a relatively high ethylene partial pressure, the losses of the ethylene and hexene, when hexene was used as the comonomer, were substantial as is exemplified in Table 1 below.
TABLE 1 ______________________________________ Commercial Ziegler-Natta Ethylene Polymerization Catalyst, 100 psi C.sub.2.sup.= Partial Pressure (lbs. loss/lb. of product) Ethylene Hexene ______________________________________ Discharge System Vent Loss 0.0082 0.0043 Dissolved Loss 0.0024 0.0408 Total Loss 0.0106 0.0451 ______________________________________
As shown in Table 1, the hexene comonomer dissolved in the product represents the largest loss from the process. Vent losses are smaller but are also significant. Assuming the raw materials cost to be 24 cents/pound of ethylene and 40 cents/pound of hexene, the total cost for unreacted monomers lost is estimated to be about 2.1 cents per pound of the product.
With the recently-developed alpha-olefin polymerization catalysts, the partial pressure of olefin monomer, such as ethylene, in the reactor is usually maintained at about 20 to about 80 psi. Since the preferred minimum operating pressure in the gas phase, fluid bed. reactor of this type is about 300 psig, an inert gas, such as nitrogen, had to be added to the reactor to compensate for the lower ethylene partial pressure. The polymerized product was discharged from the reactor and conducted to a product discharge system, typically comprised of two vessels in series, and then conveyed to a product purge vessel for removal of unreacted hydrocarbons. In the product purge vessel, the unreacted hydrocarbon reactants were removed by passing the product countercurrently to the flow of an inert purge gas, such as nitrogen, and recovering a vent gas containing the unreacted reactants. The vent gas was normally burned in a flare unit.
With such new-generation alpha-olefin polymerization catalysts, which enable the operation of the fluidized bed reactor at partial pressures of ethylene of about 20-80 psi, the reactant gas losses are reduced, as shown below in Table 2.
TABLE 2 ______________________________________ Reactant Gas Losses at Low C.sub.2.sup.= Partial Pressure (lbs. loss/lb. of product) Ethylene Hexene ______________________________________ Process operating at 60 psi C.sub.2.sup.= 0.0064 0.0261 Process operating at 25 psi C.sub.2.sup.= 0.0027 0.0080 ______________________________________
Based on the above data, potential raw material savings are 0.86 cents/pound for the process operating at 60 psi partial pressure of ethylene and 1.68 cents/pound for a process operating at 25 psi partial pressure of ethylene, as compared to the process of Table 1.
Although the advent of gas phase, fluid bed reactor processes for polymerization of alpha-olefins, such as that described by Nowlin et al, decreased the losses of the unreacted monomers because lower partial pressures of the reactants are sufficient in the fluidized bed reactor, such losses are still considerable and add substantially to the cost of the polymer products produced in the process.
Accordingly, it is a primary object of the present invention to provide an improved gas phase, fluid bed reactor process for polymerizing alpha-olefins, particularly a process which produces linear low density polyethylene and copolymers of ethylene with higher monomers, wherein the losses of the unreacted monomers are decreased.
Additional objects of the present invention will become apparent to those skilled in the art from the following description of the invention and the appended claims.